1. Field of the Invention
The present invention relates to a method of operating electro-mechanical transmission systems.
2. Description of Related Art
A known form of electro-mechanical transmission system comprises two compounded epicyclic gearsets, having an input shaft driven by a prime mover and connected to one gear element of one gearset, an output shaft providing output torque and connected to one gear element of the other gearset, two electrical machines, the rotors of which are connected to respective gear elements of the two gearsets and each of which is able to operate either as a motor or a generator and the stators of which are connected together via a controller arranged to control the flow of electrical power between the machines. Other known forms of electro-mechanical transmission systems comprise three or more compounded gearsets.
Such transmission systems have a continuously variable transmission ratio and are preferably of single regime type, that is to say they can provide all the range of gear ratios without it being necessary to provide a clutch or the like to enable switching over to a further arrangement to obtain all of the desired range of transmission ratios.
An epicyclic gearset typically comprises a sun wheel in mesh with a plurality of planet wheels, which are rotatably mounted on a common carrier and are in mesh with an annulus wheel. However, it is possible under certain circumstances for an epicyclic gearset to have only two of these gear elements, whereby one of the sun wheel, planet wheels and annulus wheel is omitted.
A transmission system of the type referred to above is disclosed in WO-A-01/94142 (Moeller) which is incorporated herein by reference. This known transmission system comprises an input shaft connected to the planet carrier of the first gearset, which is also connected to the annulus wheel of the second gearset, and an output shaft connected to the planet carrier of the second gearset. The sun wheel of the first gearset is connected to the sun wheel of the second gearset. The rotors of the first and second electrical machines are respectively connected to the annulus wheel of the first gearset and the sun wheel of the second gearset. The electrical connections of the two stators are connected together via a control system.
In use, one of the electrical machines generally acts as a generator and transfers electrical power to the other electrical machine, which acts as a motor. A proportion of the power transmitted by the transmission, typically up to about one third of the total, is handled by the electrical machines. For a given input speed, the output torque of the transmission system may be varied by controlling the power flow between the two electrical machines. The input power to the transmission system is always the output power plus the mechanical and electrical losses.
The electrical power transferred between the two electrical machines varies as the output speed varies and reaches zero at two different output speeds. When the power is zero, one of the electrical machines is stationary. Power may be transmitted between the electrical machines even at zero output speed when, e.g., tractive effort is required to pull away from rest. If no electrical power is transmitted, the transmission provides a “geared neutral”, i.e. there is no torque applied to the output shaft even though the input shaft is rotating.
The known transmission may have many different applications, which are required to be driven by a source of motive power. In the art the source of motive power is typically referred to as a prime mover. In particular the transmission system is useful as the main propulsion transmission for a motor vehicle. A rechargeable battery may be provided to enable the vehicle to be of hybrid type, that is to say the electric battery may supply electric power to one or both of the electrical machines to increase the torque on the output shaft. At those times when excess power is available, electric power may be transmitted from one or both of the electrical machines to the battery to recharge it.
As mentioned above, the transmission has a geared neutral. If, however, at zero output speed one electrical machine acts as a generator and its power is supplied to the other electrical machine, which is driven as a motor, then torque is applied at the output shaft. If the output shaft is at zero output speed, no work is performed. If one ignores the slight inefficiencies of the two electrical machines and the various meshing gearwheels, if no work is done at the output shaft, then no power is supplied to the input shaft by the vehicle engine. Accordingly, the torque applied to the output shaft is simply the sum of the torques on the two electrical machines.
The output torque available is limited by the ratings of the electrical machines. This means either that the vehicle will accelerate relatively slowly or that the electrical machines must have relatively high ratings, in which event they will be unacceptably large, heavy and expensive. This problem can of course be alleviated to an extent by utilizing the hybrid function of the vehicle, that is to say operating the controller to supply electric power from the battery to that electrical machine which is acting as a motor. This will increase the torque at that electrical machine and thus the torque on the output shaft. However, this increase is not very large and can only be for a relatively short period of time unless both the battery and the electrical machines are of a size, which will make the transmission unacceptably heavy and expensive. It is clear that one of the disadvantages of transmission systems of the type disclosed in WO-A-01/94142 is that they do not provide as much output torque as is sometimes desirable.